1. Field of the Invention
This invention relates generally to image-related devices in which one or more image transducers interact with a piece of visual-image-bearing medium; and more particularly to improvements in both character and mounting of a codestrip for use in automatic determination of transducer-carriage position in such an image-related device of large format.
A visual-image-bearing medium may be paper, vellum, or any of various types of plastic film. An image transducer is a device for creating or responding to markings on the medium. A codestrip is a graduated strip, generally disposed across an area where the medium is held, and having graduations that can be automatically sensed.
By "large format" we refer generally to devices that accommodate a piece of visual-image-bearing medium that is D size (twenty-two by thirty-four inches) or larger. As will be understood, however, our invention offers advantages, though perhaps less striking, for smaller-format medium as well.
2. Prior Art
Earlier image-related devices have provided both small and large formats. Small-format devices with which we are familiar have employed codestrips made of transparent mylar, with photographically applied indicia as graduations.
The indicia are spaced along the mylar strips with periodicity of ninety per inch, providing positioning that is theoretically good to approximately a one-hundred-eightieth of an inch through interpolation. In practice such accuracy or longterm precision is not realized because of limitations in both the strips and their mounting, as well as the carriage configuration, in the image-related devices.
In image-related devices of the sort under consideration, position of the carriage is established by a servomechanism that includes a sensor. The sensor reads graduations such as those on the codestrip just described, providing information for servocontrolled driving of the carriage to desired positions--all as set forth in, for example, U.S. Pat. Nos. 4,786,803 and 4,789,874 to Majette et al.
In such devices positional precision and accuracy are theoretically very good because all components of the carriage drive train out to and including the sensor (which is on the carriage) are inside the servocontrol loop. We have noted, however--and this recognition actually may form part of our inventive acts, with respect to the present invention--that possible sources of error still remain outside the servoloop.
Although the carriage is solid and not subject to major deformation in use, yet in contributing to transducer positional imprecision any angular cocking of the carriage that is dynamic (i.e., that changes during operation) is multiplied by an effective lever arm related to the distance between sensor and transducer. In earlier small-format devices, indeed, proximity of transducers to codestrips has not been closely controlled and has in some cases exceeded one and a half inches.
Furthermore mylar codestrips are dimensionally unstable because they expand and contract as a function of both temperature and humidity. Even more problematic is their lack of structural integrity: at elevated temperatures and mounting tensions, mylar creeps.
We digress briefly from the prior-art discussion: we have studied possible ways of adapting mylar codestrips for large-format devices, and concluded that the mylar would require costly support schemes to overcome the limitations just stated. For example, mylar strips might be placed between solid stabilizing bars in a sort of sandwich arrangement, or hung from pegs spaced across the medium, or both--awkward and expensive provisions at best.
Furthermore, due to the bulkiness of the supports, when supported in accordance with such schemes the mylar strips would not be readily positioned close to the image transducers. Poor proximity, as already explained, invites imprecisions arising in portions of the mechanism between the strip and the transducers.
For the small-format devices, however, both the precisional environment and the precisional demands are more forgiving. That is, the smaller devices are inherently more precise on an absolute basis because of the smaller carriage travel, shorter support mechanisms, etc.; and also tend to be used for types of graphics that are meant primarily for display and general information, not requiring high precision.
In a large-format device, positional error accumulates across the larger medium-holding bed, while carriage positioning elements are inherently more susceptible to vibration, cocking of the carriage and so forth. Moreover the large-format devices generally have application in relation to mechanical drawings, precision graphs and like highly technical graphics which users expect to show accurately even small differences between large dimensions.
Returning to the discussion of prior-art devices, some devices have been made using codediscs, rather than codestrips. The codediscs have been thin metallic discs held by adhesive to servomotor hubs, and carrying near-peripheral radial graduations in the form of etched orifices read by transmission-sensing detectors--analogously to the detectors used with mylar codestrips.
In such a disc an unperforated narrow peripheral border, just outward from the orifices, provides structural integrity to the codedisc. The orifices near the edge of the disc, and the unperforated border, both extend radially outward beyond the motor hub and pass through a slot in the transmission-type sensor.
Possibly such a disc is potentially more precise than a strip in determining rotation of a servomotor shaft. Indeed, as many as five hundred orifices have been provided in a disc that rotates once per inch of carriage travel; however, stating the relationships in this way can be misleading as the advantage is almost abstract or academic.
Such a system fails to account for play in the mechanism at all the various points between the shaft and the transducer or transducers, accordingly leaving transducer position known only imprecisely. (The readout from the codedisc is therefore analogous to a digital counter that has more significant figures than the precision with which it can count.)
In other words, with a hub-mounted codedisc all disturbances arising between the motor and the image transducers are outside the servocontrol loop, and the positional determination is therefore susceptible to error due to such disturbances. Here, while error control is more important than in the smaller-format mylar-strip devices discussed earlier, much more of the drive train is outside the loop than in those smaller-format devices; hence this susceptibility is doubly more severe than the problems stated earlier for those smaller devices.
In any attempt to overcome this susceptibility, such disturbances must be characterized and rejected through specifications for component manufacture. The overall result is increased product cost and nevertheless poorer positional precision.
The problems that arise in use of an elongated encoder strip are very different from those that arise in use of a circular encoder disc. Codediscs do not make use of the benefits of metal that might mitigate the problems of codestrips.
More particularly, in a hub-mounted disc there is no tension, and humidity would not be expected to have much influence on the angular dimensions of a disc--certainly not much tendency to perturb the angular relationships between different radial graduations.
A preferred solution to the problem of obtaining high precision in a codestrip should also minimize the necessity for other innovations; and should employ, to the extent possible, only such new component configurations as are very straightforward. Thus in particular we regard it as extremely desirable to employ sensors that have been used in the past for mylar codestrips, and to do so within the context of a novel electromechanical layout for a transducer carriage which--while new in important ways--preserves as many features of a previously proven layout as possible.
Sensors previously used in small-format devices with which we are familiar have been thoroughly tested and proven off-the-shelf units that read indicia on the mylar codestrips by detection of light transmitted through the strips. The sensors are slotted for passage of the codestrips, and are capable of providing information needed for interpolating carriage position between the codestrip indicia. Each slot has an open edge and a closed edge, and the active sensing elements are placed very near to the closed edge of the slot.
Similarly, transducer-carriage mechanical and electrical layouts in certain earlier units have proven performance in, e.g., microphonic and electromagnetic-interference rejection.